Greater predictability in the oral dosing of cyclosporin A (CsA) and other P4503A4 substrates may minimize the number and duration of subtherapeutic or toxic events in organ transplantation, thus improving allograft survival. The goal of our research is to test the hypothesis that the oral bioavailability of CsA in stable renal transplant recipients under steady state conditions is largely determined by both enteric and hepatic P4503A4 catalytic activity.